Method and plant for denitrifying bypass gases in a multi-stage system of mixing chambers in a plant for producing cement clinker

ABSTRACT

A method and a corresponding plant for denitrifying bypass exhaust gases in a cement clinker production plant. Raw meal is sintered in a rotary kiln and deacidified in a calciner. A rotary kiln inlet chamber is connected to the calciner directly or by a riser duct. Bypass exhaust gas is drawn off near the inlet chamber. This exhaust gas is guided into a first mixing chamber, in which the exhaust gas is cooled to between 800 and 950° C., then the exhaust gas is guided through a reaction pipeline segment, wherein the dwell time is between 0.5 and 3 seconds and ammonia, aqueous ammonia solution, or ammonia-releasing substances are injected for denitrification. Then the exhaust gas is guided into a second mixing chamber, in which the exhaust gas is cooled to between 150 to 250° C. Then the exhaust gas is guided to a filter for dust removal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.15/554,163, filed on Aug. 28, 2017, which is a national stage ofInternational Patent Application No. PCT/EP2016/054381, filed on Mar. 2,2016, which claims the benefit of the German patent application No. 102015 002 688.7 filed on Mar. 4, 2015, the entire disclosures of whichare incorporated herein by way of reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for the denitrification of bypassexhaust gases in a plant for producing cement clinker, wherein the planthas a rotary kiln for the sintering of raw meal to cement clinker andhas a calciner for the deacidification of the raw meal, downstream ofthe rotary kiln in the kiln exhaust gas flow direction, the rotary kilnhas a rotary kiln inlet chamber which is connected directly or via akiln riser duct to the calciner, and the bypass exhaust gas is drawn offin the region of the rotary kiln inlet chamber. The invention furtherrelates to a corresponding plant for the denitrification of bypassexhaust gases in the production of cement clinker, comprising a rotarykiln for the sintering of raw meal to cement clinker, the rotary kilnhaving a rotary kiln inlet chamber, a calciner for the deacidificationof the raw meal, the rotary kiln inlet chamber being connected directlyor via a kiln riser duct to the calciner, and a takeoff device fordrawing off the bypass exhaust gases from the region of the rotary kilninlet chamber.

Within the overall operation of cement production, plants are employedin which silicate-containing and carbonate-containing raw meal issintered to cement clinker in a rotary kiln. The sintering in the rotarykiln, which proceeds at temperatures of up to around 1450° C., producesflue gases which, as hot exhaust gases, leave the rotary kiln in thedirection opposite to the flow of material, through the inlet chamber ofthe rotary kiln. In the normal instance, the kiln exhaust gases thenflow into a calcining zone, in which the raw meal is deacidified. Thecalcining zone is most often formed in a kiln riser duct or in acalciner, or in a kiln riser duct and a calciner downstream (in the gasflow direction). The flue gas then flows further into a heat exchanger,designed, for example, as a multistage cyclone heat exchanger, whichserves for the preheating of the raw meal. A problem affecting theoperation of cement clinker production is the formation and/or releaseof a series of pollutants. In particular, on account of the hightemperatures of the burner flames (about 1800° C. to 2000° C.), nitrogenoxides (NOx) are formed in the rotary kiln by combustion of the nitrogenwhich is contained within the atmospheric air. The fuel required aswell, especially when using secondary fuels such as the replacementfuels obtained from waste, is another source of nitrogen oxides. Giventhat nitrogen oxides have adverse consequences for people and theenvironment—as a cause of acid rain, and through breakdown of ozone inthe stratosphere, for example—there are strict limits on the emission ofnitrogen oxides into the atmospheric environment. In the course ofcement production, therefore, methods must be employed for thedenitrification of the flue gases.

A further problem is that the raw materials for cement clinkerproduction, and also the fuels employed, especially secondary fuels,contain by-constituents (alkali metal compounds, chlorine, sulfurcompounds, heavy metals, etc.) which not only may be detrimental to thequality of the combustion process and/or of the cement clinker, but mayalso form deleterious substance circuits within the plant for cementclinker production. In the rotary kiln, for example, there isevaporation of alkali metal sulfates and alkali metal chloridecompounds, e.g., potassium chloride (KCl). With the kiln exhaust gas,these compounds pass through the kiln inlet chamber into the calciningzone and the heat exchanger, and they condense on the raw meal particlesin the cooler regions, and pass with the material stream back into therotary kiln, where they evaporate again. Further to the disadvantages ofsuch substance circuits for the cement clinker and the combustionprocess, rapid cooling and condensation of these compounds give rise,through solidification, to caking on the walls of the cooler sections ofthe circuit, which may cause the plant to become blocked over time.

For the purpose of suppressing substance circuits of this kind in plantsfor cement clinker production, and for reducing the level ofcircuit-forming substances, the patent specification DE 197 18 259 B4discloses drawing off, as a bypass, a part of the flue gas that flows askiln exhaust gas from the rotary kiln, in the region of the rotary kilninlet chamber. The phrase “in the region of the rotary kiln inletchamber,” here and below, refers consistently to removal from the rotarykiln inlet chamber or else to removal from the lower end of any kilnriser duct there may be. Even the bypass exhaust gas, however, containsa higher level of nitrogen oxides, and so flue gas denitrification mustbe performed for the bypass exhaust gas as well.

One widespread method for the denitrification of flue gases involvesfeeding the NOx-affected flue gases with an aqueous ammonia solution,ammonia (NH3) or ammonia-releasing compounds in a reaction space (see,for instance, the proposal contained in EP 0 854 339 A1).Denitrification then proceeds by the process of selective non-catalyticreduction (SNCR), in which ammonia is converted by thermolysis with thenitrogen oxides into nitrogen and water. These reactions proceedpreferably in a temperature window from 800° C. to more than 950° C. Foreffective implementation, furthermore, it is necessary to realize atimespan which requires precise establishment, and at any rate a minimumtime, for the processes within the reaction space. In the case of thedesired denitrification of bypass exhaust gas, however, denitrificationby the SNCR process proves to be problematic, since the temperatures ofthe bypass exhaust gas drawn off are too high and, in addition,compliance with the residence time in the reaction space imposesexacting requirements on the operating regime. It is true that thetemperature of the exhaust gases in the kiln falls from up to about1250° C. on entry into the rotary kiln inlet chamber and the lower partof any kiln riser duct that may be present, and yet the gas temperaturesof around 1150° C. which still prevail at this point are still so highthat reducing agents added would undergo combustion.

One known procedure (DE 197 18 259 B4, DE 199 10 927 A1) is to carry outrapid cooling, preferably to just a few hundred ° C., of the hot bypassexhaust gas stream in mixing chambers, in which a cooling medium such aswater or air is injected and is mixed as extensively as possible withthe gas stream. This does have the advantage that evaporated substanceswhich are drawn off from the pollutant circuits condense on the surfacesof the particulate solids and can then be removed together with thesesolids by means of dust filters. For effective denitrification, however,this operation is not suitable.

The German patent application with the number 10 2013 016 701.9discloses a method for the denitrification of bypass exhaust gases in aplant for producing cement clinker by initially cooling the bypassexhaust gas to temperatures between 260° C. and 400° C. in a mixingchamber, for instance. This is followed by the feeding of the cooledbypass exhaust gas with substances containing ammonia, containing ureaand/or containing ammonium. A consequence of this is that the nitrogenoxides are subject to selective chemical reduction over a catalyst whichis present in a ceramic filter arrangement and/or which immediatelyfollows the ceramic filter arrangement, in the presence of thesubstances containing ammonia, urea and/or ammonium. In terms of method,therefore, the denitrification in this case is based on the process ofselective catalytic reduction (SCR). Capital costs and operating costsfor the catalyst and/or catalytic filter required in the case of SCR,however, are comparatively high. Especially when volume flows of bypassexhaust gas are comparatively low, this process may prove economicallyto be not very advantageous, thus illustrating the advantageous natureof alternative procedures.

SUMMARY OF THE INVENTION

It is an object of the invention, therefore, to specify an effectivemethod for the denitrification of bypass exhaust gases in a plant forproducing cement clinker that does not involve the use of SCR catalysts.A further part of the object of the invention is to propose a plant forthe denitrification of bypass exhaust gases that corresponds to themethod.

An object of the invention is achieved by passing of the bypass exhaustgas into a first mixing chamber, the bypass exhaust gas being cooled inthe first mixing chamber to a temperature of between 800° C. and 950°C., passing of the bypass exhaust gas from the first mixing chamberthrough a reaction section which is disposed in a conduit, the residencetime of the bypass exhaust gas in the reaction section being between 0.5s and 3 s, and ammonia, aqueous ammonia solution or ammonia-releasingsubstances being injected into the reaction section for denitrificationof the bypass exhaust gas in accordance with the process of selectivenon-catalytic reduction (SNCR), passing of the bypass exhaust gas fromthe reaction section into a second mixing chamber, the bypass exhaustgas being cooled in the second mixing chamber to a temperature ofbetween 150° C. and 250° C., and by passing of the bypass exhaust gasfrom the second mixing chamber to at least one filter for the dedustingof the bypass exhaust gas. An object of the invention is furtherachieved by a plant corresponding to this method, for thedenitrification of bypass exhaust gases in the production of cementclinker.

Following diversion of the bypass exhaust gas stream, therefore, theinvention provides a multistage bypass system wherein the hot bypassexhaust gas is first cooled in a first mixing chamber to a temperatureof between 800° C. and 950° C. The temperature window therebyestablished in the NOx-affected bypass exhaust gas is one which isfavorable for SNCR. In comparison to the stated conventional methods, inwhich the bypass exhaust gas temperature is lowered in one steptypically to temperatures of 200° C. to 400° C., then, the temperatureshere are not lowered in one step. To do so would be to rule out an SNCRapproach. By appropriate internal construction of the mixing chamber,particularly of the path length for the gas stream, and by appropriatemetering of the injected cooling media, the skilled person insteadachieves the desired cooling to 800° C. to 950° C. The method iscontinued in the next step by the denitrification itself, namely byinjection of ammonia, aqueous ammonia solution or ammonia-releasingsubstances into a reaction section which is disposed in a reactorcavity, implemented primarily as a conduit. The denitrification here isbased on the SNCR method, and thus in comparison to the SCR method doesnot necessitate any costly and difficult-to-operate catalysts. Thefurther parameter needed for effective denitrification, namely theresidence time of the bypass exhaust gas in the reaction section (0.5 to3 seconds), is established through the dimensioning of the conduit, thusmore particularly by the length thereof and also the cross-sectionalarea thereof. Accordingly, there is extensive denitrification withoutexcessive construction volumes, on the one hand, and with avoidance ofinadequate flow rates of the bypass exhaust gas, on the other hand. Theoperating regime according to the invention then provides for cooling ofthe denitrified bypass exhaust gas in a second mixing chamber. Thetarget final temperature of between 150° C. and 250° C. is achieved hereby rapid cooling, and so other pollutants as well, as in the case ofconventional methods which operate with rapid cooling (quenching), arecondensed on the dust and are separated off together with theparticulate solids at a downstream filter. Examples of suitable filtersare fabric filters/cloth filters/bag filters.

One preferred embodiment provides for the residence time of the bypassexhaust gas in the reaction section to be between one and two seconds. Apassage time of this kind for the bypass exhaust gas proves to beideally suited to achieving efficient, far-reaching denitrification ofthe bypass exhaust gas by selective non-catalytic reduction (SNCR), atthe temperature levels of 800° C. to 950° C. which are established bycooling in the preceding method step in the first mixing chamber,without having to equip the reaction section for an even longerresidence time of the bypass exhaust gas. Since the reaction section isdesigned as a zone in a conduit or in a conduit-like cavity, theresidence time may be determined through the dimensioning, i.e., throughthe dimensions of the conduit on the basis of the typical values in thespecific plant in question. For fine adjustments or for compensating forfluctuations in the flow rate of the bypass exhaust gas in the operationof the plant for producing cement clinker, it is possible, furthermore,to adapt the withdrawal of the volume flow of the bypass exhaust gas orto vary the flow rate in the conduit with the reaction section, byaltering the cross-sectional area, for instance.

An embodiment of the invention provides for the bypass exhaust gas to becooled in the second mixing chamber preferably to a temperature ofbetween 180° C. and 220° C. The primary consideration here is rapidcooling of the bypass exhaust gas. Within this temperature window,formation of dioxins (chlorinated and polychlorinated dibenzodioxins)and furans (chlorinated and polychlorinated dibenzofurans) is largelyprevented. Furthermore, the rapid attainment of this final temperatureis the most effective way of achieving condensation of pollutants on thedust, which is then deposited downstream in the filter.

In accordance with the invention, the system for purifying the bypassexhaust gas that has been drawn off comprises a multistage system whichcomprises two mixing chambers; as and when necessary, further mixingchambers may also be provided. In the case of conventional mixingchambers, a cooling medium, typically air or water, is injected into themixing chamber, and mixes with the bypass exhaust gas throughappropriate steering of the gas stream in the mixing chamber, andrapidly cools the bypass exhaust gas in the process. As a preferredembodiment of the invention, accordingly, provision is made for thecooling medium injected into the second mixing chamber to comprise wateror fresh air, or a combination of water and fresh air. According to oneembodiment, water or fresh air, or a combination of water and fresh air,is also injected into the first mixing chamber, which may also be termeda premixing chamber. The injection of water into a mixing chamber heretakes place advantageously with atomization in a spray, using two-fluidnozzles, for instance. In principle, the injection of fresh air (forinstance atmospheric ambient air) and/or water may occur at variouspoints distributed over the mixing chamber wall, the aim being formaximum mixing and uniform cooling. A particular embodiment of theinvention introduces fresh air, water and cold raw meal or any desiredcombination, in other words one, two or three components thereof, as acooling medium into the first mixing chamber. The injection of cold mealhas the additional advantage of reducing the sulfur dioxide content ofthe bypass exhaust gas; the raw meal, with the surface area of itsparticles, is available as a sorbent for pollutant purification in thesubsequent course of the method as well. In the first mixing chamber,furthermore, hot meal, if not yet deacidified, can also be used as acooling medium and/or as a constituent in the stated cooling mediacombination.

In order to maintain continuous, uniform gas flow within the bypasssystem, suction blowers or fans may be used. In one embodiment of theinvention, provision is made for a fan to be disposed downstream of theat least one filter in the gas flow direction. This fan may also be amotor-driven compressor. In normal circumstances, the purified bypassexhaust gas is subsequently given off into the environment via achimney, although is also available to be fed back in, whereappropriate, at a suitable point within the overall process of cementproduction.

In a further embodiment of the invention, the reaction section is fedwith at least one sorbent for the additional, pollutant-removingpurification of the bypass exhaust gas, for which purpose at least onedevice for feeding in at least one such sorbent is disposed in theregion of the conduit that constitutes the reaction section.Contemplated here in principle are all chemical compounds or sorbentswhich within the time of passage of the bypass exhaust gas through thereaction section, and at the temperatures prevailing therein, aresuitable for ensuring reduction in the level of pollutants. In thiscase, it is possible for the temperature which is needed for theparticular reaction to be established in a defined way in the firstmixing chamber. Here, for example, calcium compounds such as calciumcarbonate may contribute to desulfurization, and also to reduction offurther acidic constituents in the flue gas-like bypass exhaust gas.Activated carbon provides large surface areas for the adsorption ofpollutants, such as of heavy metals like mercury. Removal then takesplace within the filter unit.

The invention here is oriented to the implementation of SNCRdenitrification, but is not confined to this important case. Theconstruction of the bypass system makes it possible in principle for atemperature window to be established by cooling of a hot exhaust gasstream in a first mixing chamber, and for a time window for theresidence of the gas stream in a reaction section to be establishedsubsequently. Temperature intervals and time intervals here may also berealized in such a way that, when suitable chemical compounds and/orsorbents are injected into the reaction section, other reactions and/ormethods for pollutant-removing purification of the exhaust gas arefavored. The skilled person is able here to transpose the principle tothe particular group of pollutants that is the primary object forremoval from the exhaust gas stream, on the basis of limit values, forinstance.

Furthermore, the invention is not confined to plants which have acalciner, since the bypass gases are drawn off in the region of therotary kiln inlet chamber itself. The process of the invention ofpurification of bypass exhaust gas in the bypass mixing chamber systemproposed can instead be employed universally for kiln exhaust gaspurification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated with the FIGURE that follows.

The FIGURE shows a schematic representation of the method of theinvention for the denitrification of bypass exhaust gases in a plant forproducing cement clinker.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the FIGURE it is evident schematically that flue gas 1 flows into therotary kiln inlet chamber 5 from a rotary kiln 2, in which raw meal 3 issintered to form cement clinker which is then cooled in a clinker cooler4. Following this in the direction of gas flow, in the working exampledepicted, are a kiln riser duct 6 and a calciner 7 for thedeacidification of the raw meal 3. A fraction of the flue gas 1 (kilnexhaust gas) flows through kiln riser duct 6 and calciner 7 into theheat exchanger 8 (presently a multistage cyclone heat exchanger), whichserves for the preheating of the raw meal 3 for cement production.

In accordance with the invention, a part of the flue gas stream 1emerging from the rotary kiln 2 is drawn off as bypass exhaust gas 9 inthe region of the rotary kiln inlet chamber 5, i.e., from the rotarykiln inlet chamber 5 or from the kiln riser duct 6. The bypass exhaustgas 9, which initially on emergence from the rotary kiln 2 hastemperatures typically of around 1200° C. to 1300° C., and of around1000° C. to 1200° C. at the takeoff location, is passed to the firstmixing chamber 10. Cooling media fed into the first mixing chamber 10may be fresh air 11, water 12 or cold meal 12 a, or else hot meal, andalso any desired mixtures of these. In the working example depicted,atmospheric fresh air 11, water 12 and cold meal 12 a are injected, withcold raw meal—that is, raw meal which has not already been heated in theheat exchanger—is advantageous particularly for a reduction in theamount of sulfur dioxide in the bypass exhaust gas. In the first mixingchamber 10, by extensive mixing of the bypass exhaust gas 9 with thecooling media, the bypass exhaust gas 9 is cooled to temperatures ofbetween 800° C. and 950° C.

After emerging from the first mixing chamber 10, i.e., after the firstcooling stage, the bypass exhaust gas 9 enters a conduit 13. Injectedinto the conduit 13 are ammonia, aqueous ammonia solution orammonia-releasing substances 14. The flow rate of the bypass exhaust gasand the dimensioning of the conduit 13 are matched to one another insuch a way as to result in a residence time for the bypass exhaust gas 9in the conduit 13 of 0.5 s to 3 s, preferably between 1 s and 2 s. Thetemperature conditions of the bypass exhaust gas 9 and the residencetime are therefore established in such a way that there is effectivedenitrification of the bypass exhaust gas by the process of selectivenon-catalytic reduction (SNCR) over a reaction section 15 which isformed within the interior of the conduit. Ammonia 14 here is convertedby thermolysis into nitrogen and water. The temperatures of the bypassexhaust gas 9 without cooling or before cooling in the first mixingchamber 10 would be too high for such an SNCR, since the reducing agentswould undergo combustion at such high temperatures. Additional feedingof sorbents which ensure further pollutant-removing purification of thebypass exhaust gas 9 is possible in the region of the reaction section15.

Following completed denitrification by SNCR in the reaction section 15,the bypass exhaust gas 9 is passed into a second mixing chamber 16. Inthe second mixing chamber 16 it is rapidly cooled to the desired finaltemperature of between 150° C. and 250° C., preferably between 180° C.and 220° C. This second cooling stage is accomplished by injection ofwater 12 and/or fresh air 11 into the second mixing chamber 16. Rapidcooling to these temperatures minimizes the formation of dioxins andfurans and leads to condensation of pollutants on the dust. The bypassexhaust gas 9 thus conditioned is subsequently dedusted in at least onefilter 17. Suitability here is possessed by fabric filters/clothfilters/bag filters, and the use of electrostatic filters, and also acombination of different types of filter in series, may also beadvantageous. In the working example, the purified bypass exhaust gas 9subsequently passes through a fan 18, and is drawn off by a chimney 19and released into the environment. In the bypass system as a whole, suchas especially in the mixing chambers, effective commixing and acorrespondingly uniform temperature field, and also a suitable bypassexhaust gas flow rate, are important for effective method steps.Depending on the arrangement and the associated path lengths, it may beadvantageous in particular plants to provide internals in the gaspathway that ensure effective commixing, and also to provide additionalfans in the bypass section, which introduce air into the bypass exhaustgas flow through continuous or discontinuous operation.

As a result of the construction according to the invention, alteredrelative to conventional procedures, and by the altered operatingregime, the means of denitrification of the bypass exhaust gases is alsoeffective and is also favorable in terms of acquisition and inoperation, and removes the need for SCR catalysts to be used.

As is apparent from the foregoing specification, the invention issusceptible of being embodied with various alterations and modificationswhich may differ particularly from those that have been described in thepreceding specification and description. It should be understood that Iwish to embody within the scope of the patent warranted hereon all suchmodifications as reasonably and properly come within the scope of mycontribution to the art.

Claimed is:
 1. A plant for the denitrification of bypass exhaust gases in the production of cement clinker, comprising: a rotary kiln for the sintering of raw meal to cement clinker, the rotary kiln having a rotary kiln inlet chamber, a calciner for the deacidification of the raw meal, the rotary kiln inlet chamber being connected directly or via a kiln riser duct to the calciner, and a takeoff device for drawing off the bypass exhaust gases from the region of the rotary kiln inlet chamber, wherein a first mixing chamber is provided for the cooling of the bypass exhaust gas, drawn off from the region of the rotary kiln inlet chamber, to a temperature of between 800° C. and 950° C. so that chloride compounds remain in a gaseous phase after the bypass exhaust gas is cooled, wherein downstream of the first mixing chamber in the gas flow direction, a reaction section is provided which is disposed in a conduit, wherein the conduit has dimensioning such that the residence time of the bypass exhaust gas in the reaction section is between 0.5 s and 3 s, and wherein in the region of the conduit, at least one device is provided for the injection of ammonia, aqueous ammonia solution or ammonia-releasing substances into the reaction section for the denitrification of the bypass exhaust gas by the process of selective non-catalytic reduction (SNCR), wherein downstream of the reaction section, a second mixing chamber is disposed, for the cooling of the bypass exhaust gas to a temperature of between 150° C. and 250° C. to remove chloride compounds from the gaseous phase by deposition, and wherein downstream of the second mixing chamber at least one filter is disposed for the dedusting of the bypass exhaust gas.
 2. The plant as claimed in claim 1, wherein the conduit is dimensioned such that the residence time of the bypass exhaust gas in the reaction section is between 1 s and 2 s.
 3. The plant as claimed in claim 1, wherein downstream of the at least one filter in the gas flow direction there is disposed a fan.
 4. The plant as claimed in claim 1, wherein in the region of the conduit, at least one device is provided for feeding at least one sorbent into the reaction section for additional, pollutant-removing purification of the bypass exhaust gas.
 5. An apparatus for the denitrification of bypass exhaust gases in the production of cement clinker, comprising: a rotary kiln configured to sinter raw meal and provide cement clinker, the rotary kiln having a rotary kiln inlet chamber; a calciner configured to deacidify raw meal, the rotary kiln inlet chamber being connected directly or via a kiln riser duct to the calciner; a first mixing chamber configured to receive bypass exhaust gas from the rotary kiln inlet chamber and cool the bypass exhaust gas to a temperature of between 800° C. and 950° C. so that chloride compounds remain in a gaseous phase after the bypass exhaust gas is cooled; a reactor, configured to receive the bypass exhaust gas after the bypass exhaust gas is cooled in the first mixing chamber, the reactor disposed in a conduit and dimensioned such that the residence time of the bypass exhaust gas in the conduit is between 0.5 s and 3 s, at least one device configured to inject ammonia, aqueous ammonia solution or ammonia-releasing substances into the reactor for the denitrification of the bypass exhaust gas by the process of selective non-catalytic reduction (SNCR), a second mixing chamber configured to receive the bypass exhaust gas and ammonia, aqueous ammonia solution or ammonia-releasing substances from the reactor and to cool the bypass exhaust gas to a temperature of between 150° C. and 250° C. to remove chloride compounds by deposition, and at least one filter, downstream of the second mixing chamber in the gas flow direction, for dedusting the bypass exhaust gas after it has been cooled in the second mixing chamber. 